Electric direct-acting die clamping unit of an injection molding machine

ABSTRACT

An object of the present invention is to suppress clamping force from sharply increasing when dies are clamped in an electric direct-acting die clamping unit of an injection molding machine. A stationary platen and a back plate are arranged at both ends of a base. A movable platen is arranged to face a stationary platen. A stationary die is attached to the stationary platen, whereas a movable die is attached to the movable platen. The tip portion of a threaded rod of a ball screw is fixed at the back surface of the movable platen via a load cell. A nut of the ball screw is accommodated in a housing and supported by the bearing. The housing is connected to the front surface of the back plate via the coil spring. When the nut is rotated the threaded rod is driven to move the movable platen back and forth.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-136461, filed May 7, 2001, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a die clamping unit of an injection-molding machine, and more specifically, to the structure of an electric direct-acting die clamping unit directly transmitting thrust from a driving source to a movable platen without using a toggle mechanism.

[0004] 2. Description of the Related Art

[0005]FIG. 2 shows a schematic structure of a conventional electric direct-acting die clamping unit.

[0006] A stationary platen 3 and a back plate 62 are arranged at both ends of a base 1 so as to face each other. A movable platen 4 is arranged in the front of the back plate 62 so as to face the stationary platen 3. A stationary die 5 is attached to the front surface of the stationary platen 3, whereas a movable die 6 is attached to the front surface of the movable platen 4.

[0007] The back plate 62 and the stationary platen 3 are connected by four tie rods 67. A through-hole is formed at each of the four corners of the movable platen 4. The four tie rods 67 are passed through the corresponding through-holes. The movable platen 4 slidably moves back and forth along the tie rods 67, that is, in the die clamping and unclamping directions.

[0008] The movable platen 4 is connected to the front surface of the back plate 62 via a ball screw 70. More specifically, a load cell 75 is fixed to the back surface of the movable platen 4. The tip portion of a threaded rod 71 of a ball screw is fixed to the back surface of the load cell 75. A nut 72 of the ball screw is rotatably supported by the back plate 62 via a bearing 77.

[0009] A motor 80 is arranged under the base 1. A pulley 82 is attached to the end surface (at the side close to the movable die) of the nut 72. Another pulley 81 is attached to the shaft of the motor 80. A timing belt 83 goes around the two pulleys 81 and 82. When the motor 80 rotates the nut 72, the threaded rod 71 moves back and forth along its axis, thereby moving the movable platen 4 back and forth along the tie rods 67.

[0010] As mentioned above, in the conventional electric direct-acting die clamping unit, the movable platen 4 is moved by means of the ball screw 70 to clamp and unclamp the dies.

[0011] (A Problem of a Conventional Direct-Acting Die Clamping Unit)

[0012] When the dies are clamped by the conventional direct-acting die clamping unit, the reaction force is received only by four tie rods 67. The tie rods 67 are rigid since the total sectional area of the tie rods is large. Therefore, even if the reaction force is applied to the tie rods, the tie rods do not stretch so much. Therefore, the die clamping force (i.e., compressive stress applied to the die surfaces) sharply increases immediately after the die surface of the stationary die 5 comes into contact with that of the movable die 6. As a result, the overshoot of the die clamping force occurs. Therefore, it becomes difficult to control the clamping force accurately.

[0013] Furthermore, since an excessively large load is applied to structural parts of the die-clamping unit, the life of parts are short.

BRIEF SUMMARY OF THE INVENTION

[0014] The present invention has been made in view of the problems associated with the electric direct-acting die clamping unit of a conventional injection-molding machine. An object of the present invention is to provide an electric direct-acting die clamping unit capable of accurately controlling a die-clamping force by suppressing the die-clamping force from sharply increasing, thereby preventing the overshoot of the die-claming force.

[0015] According to the present invention, there is provided an electric direct-acting die clamping unit of an injection molding machine, comprising:

[0016] a base;

[0017] a back plate fixed at one of the ends of the base;

[0018] a stationary platen fixed at the other end of the base and holding a stationary die;

[0019] a movable platen arranged on the base to face the stationary platen and holding a movable die;

[0020] a ball screw having a threaded rod and a nut, the tip of the threaded rod is fixed at the back surface of the movable platen;

[0021] a housing rotatably supporting the nut;

[0022] an elastic member connecting between the back plate and the housing and contractable in the directions of clamping and unclamping the dies; and

[0023] a motor rotating the nut of the ball screw.

[0024] Preferably, a coil spring is used as the elastic member.

[0025] Now, the operation of the electric direct-acting die clamping unit of the present invention will be explained.

[0026] The movable platen is moved forward and backward to the stationary platen by moving the ball screw. When the dies are clamped, the movable platen is moved toward the stationary platen to bring the die surfaces of both dies into contact with each other. At this time, the reaction force against the die clamping force (compressive stress applied to the die surfaces) is transmitted to the back plate via the ball screw and the elastic member. As a result, the threaded rod elastically contracts, and simultaneously the elastic member deforms in the contraction direction.

[0027] The spring constant of the elastic member used herein is appropriately set to permit the elastic member to contract relatively more than the threaded rod contracts. By virtue of this, the die clamping force increases gently when the die surfaces are brought into contact with each other by clamping the dies. In this manner, the die clamping force can be controlled accurately. As a result, according to the electric direct-acting die clamping unit of the injection molding machine of the present invention, the overshoot of the die clamping force can be prevented, thereby controlling the clamping force accurately.

[0028] Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0029] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.

[0030]FIG. 1 is a front view showing a schematic structure of an electric direct-acting die clamping unit of the present invention; and

[0031]FIG. 2 is a view showing a schematic structure of a conventional electric direct-acting die clamping unit.

DETAILED DESCRIPTION OF THE INVENTION

[0032]FIG. 1 shows a schematic structure of the electric direct-acting die clamping unit of an injection-molding machine according to the present invention. The unit shown in the figures has a base 1, a back plate 2, a stationary platen 3, a movable platen 4, a stationary die 5, a movable die 6, a housing 8, a coil spring 9 (elastic member), a ball screw 10, a threaded rod 11 of the ball screw, and a nut 12 of the ball screw.

[0033] The stationary platen 3 and the back plate 2 are respectively arranged at both ends of the base 1 so as to face each other. The movable platen 4 is arranged in the front of the back plate 2 so as to face the stationary platen 3. The stationary die 5 is attached to the front surface of the stationary platen 3, whereas the movable die 6 is attached to the front surface of the movable platen 4. The movable platen 4 is designed to slide on the base 1, thereby moving back and forth (that is, in the directions of clamping and unclamping the dies).

[0034] The movable platen 4 is connected to the front surface of the back plate 2 via the coil spring 9 and the ball screw 10, as follows. The housing 8 is arranged in the front of the back plate 2. The nut 12 of the ball screw is accommodated in the housing 8. The housing 8 is connected to the front surface of the back plate 2 by a pair of coil springs placed upper and lower portions. The coil spring 9 contracts along the threaded rod 11 of the ball screw. The nut 12 of the ball screw is rotatably supported by the housing 8 via a bearing 17. A load cell 15 is fixed to the back surface of the movable platen 4. The tip portion of the threaded rod 11 is fixed to the back surface of the load cell 15.

[0035] A motor 20 is arranged under the base 1. A pulley 22 is attached to the end surface (close to the movable platen) of the nut 12 of the ball screw. A pulley 21 is attached to the shaft of the motor 20. A timing belt 23 goes around the pulleys 21 and 22. When the nut 12 is rotated by the motor 20, the threaded rod 11 moves along its axis to move the movable platen 4 forward and backward on the base 1. In this manner, the dies are clamped and unclamped by moving the ball screw 10.

[0036] As mentioned above, in the electric direct-acting die clamping unit, the ball screw 10 is attached to the front surface of the back plate 2 via the coil spring 9. When dies are clamped, a ball screw 10 is driven to move the movable platen 4 toward the stationary platen 3. As a result, the stationary die 5 comes into contact with the movable die 6. At this time, the reaction force against the clamping force (compressive stress applied to the die surfaces) is transmitted to the back plate 2 through the threaded rod 11, nut 12, housing 8, and coil spring 9. As a result, the threaded rod 11 elastically contracts and the coil spring 9 contracts also.

[0037] The spring constant of the coil spring 9 is appropriately set so as to permit the coil spring 9 to contract relatively more than the threaded rod 11 contracts. By virtue of this, the die clamping force increases gently when the die surfaces are brought into contact with each other by clamping the dies. In this manner, the die clamping force can be controlled accurately. As a result, the overshoot of the clamping force can be prevented, thereby controlling the clamping force more accurately.

[0038] In the aforementioned embodiment, a pair of coil springs 9 are arranged above and below the threaded rod 11 equidistant from the rod. Alternatively, four coil springs are arranged at four corners respectively, or a pair of coil springs are arranged at each of diagonally-opposed corner positions. The arrangement of the coil springs may be modified in various ways as mentioned above.

[0039] In the aforementioned embodiment, a tie rod is not used for supporting the movable platen 4. However, the present invention may be applied to the die clamping unit where two or four tie rods are used.

[0040] According to the electric direct-acting die clamping unit of the injection-molding machine of the present invention, the ball screw, which directly moves the movable platen, is attached to the front surface of the back plate via the elastic member. Therefore, the die clamping force increases gently when the die surfaces are brought into contact with each other by clamming the dies. Accordingly, the die clamping force can be controlled accurately. As a result, the overshoot of the clamping force can be prevented, thereby controlling the clamping force more accurately. Furthermore, an excessive load cannot be applied on the parts of the die-clamping unit, so that the life of the parts can be extended.

[0041] According to the electric direct-acting clamping unit of the injection-molding machine of the present invention, it is possible to omit the tie-rods usually used in a conventional die clamping unit. Therefore, the number of parts of the machine can be reduced, simplifying the structure of the machine. In addition, the operator can more easily get access to the dies, compared to the conventional machine.

[0042] Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

What is claimed is:
 1. An electric direct-acting die clamping unit of an injection molding machine comprising: a base; a back plate fixed at one of the ends of the base; a stationary platen fixed at the other end of the base and holding a stationary die; a movable die platen arranged on the base to face the stationary platen and holding a movable die; a ball screw having a threaded rod and a nut, the tip of the threaded rod being fixed at the back surface of the movable platen; a housing rotatably supporting the nut; an elastic member connecting between the back plate and the housing and contractable in the directions of clamping and unclamping the dies; and a motor rotating the nut of the ball screw.
 2. An electric direct-acting clamping unit of an injection-molding machine according to claim 1, wherein said elastic member is a coil spring. 